Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and is favorable for cameras for broadcasting and filmmaking, home-use video cameras, and the like in which the amounts of displacement of the in-focus position due to manufacturing errors and changes in temperature are well corrected.
Description of the Related Art
Generally, a zoom lens for use in shooting television programs, shooting films, or doing the like includes a focus lens unit for focusing, a magnification lens unit including a variator for magnification and a compensator for correction of image plane displacement resulting from magnification, and an imaging lens unit for imaging, in this order from the object side. Note that for some lenses for broadcasting, an extender unit is disposed in the imaging lens unit so as to be switchable in position.
Further, to bring a subject into focus, manual focusing is performed in which the focus lens unit is driven in the optical axis direction by mechanically transmitting a manual operation to the focus lens unit or by electrically driving the focus lens unit in accordance with the manual operation.
The shooting of television programs and films often involves not only bringing a subject into focus but also purposely bringing a subject out of focus as a visual effect. For this reason, the above manual focusing is performed to allow focusing at the shooter's will.
Also, in sports broadcasting and studio shooting, quick zoom operation is required. For this reason, a zoom cam mechanism has been commonly used which moves a magnification lens unit in the optical axis direction via rotation of a cam cylinder.
Zoom lenses that are driven by manual focus and zoom cam mechanisms as mentioned above have a problem in that the in-focus position in a middle zoom range is displaced by manufacturing variations in refractive index of the magnification lens unit, intervals therein, and the like and changes in temperature.
Further, there has been a problem in that the in-focus position is displaced by a change in spherical aberration between when the stop is opened and when the stop is closed, a change in spherical aberration due to the positioning of the focus lens, a change in posture, and so on.
In order to solve the above problems, it is necessary to reduce manufacturing variations and perform precise assembly adjustment. However, with the spread of high-definition cameras in recent years, the resolving power has become higher and the depth of focus has become shallower than those of conventional HD cameras. Accordingly, it has become difficult to keep the above-mentioned displacement of the in-focus position within the depth of focus.
In order to address such a problem, Japanese Patent Application Laid-Open No. H10-186209, for example, proposes the following method. Specifically, at the time of assembling a lens, displacements of the in-focus position are figured out in advance which are caused by changes in zoom position, in stop position, and in position of an extender. Then, when the zoom position, the stop position, or the position of the extender is changed while the lens is used, the amount of shift of the in-focus position is figured out from a lookup table or a calculation equation on the basis of that change, and a master lens unit (imaging lens unit) is driven to cancel out the amount of that displacement.
Japanese Patent Application Laid-Open No. H10-186209, however, does not at all disclose any specific lens arrangement or configuration for achieving quick zoom operation and good optical performance while correcting displacement of the in-focus position.